The related patent applications describe two-stroke cycle, compression-ignition, uniflow-scavenged, opposed-piston engines in which pairs of pistons move in opposition in the bores of ported cylinders. A two-stroke cycle opposed-piston engine completes a cycle of engine operation with two strokes of a pair of opposed pistons. During a compression stroke, as the pistons begin to move toward each other, charge air is admitted into the cylinder, between the end surfaces of the pistons. As the pistons approach respective top dead center (“TDC”) locations to form a combustion chamber the charge air is increasingly compressed between the approaching end surfaces. When the end surfaces are closest to each other, near the end of the compression stroke, a minimum combustion chamber volume (“minimum volume”) occurs. Fuel injected directly into the cylinder mixes with the compressed charge air. Combustion is initiated when the compressed air reaches temperature and pressure levels that cause the fuel to begin to burn; this is called “compression ignition”. Combustion timing is frequently referenced to minimum volume. In some instances, injection occurs at or near minimum volume; in other instances, injection may occur before minimum volume. In any case, in response to combustion the pistons reverse direction and move away from each other in a power stroke. During a power stroke, the pistons move toward bottom dead center (“BDC”) locations in the bore. As the pistons reciprocate between top and bottom dead center locations they open and close ports formed in respective intake and exhaust locations of the cylinder in timed sequences that control the flow of charge air into, and exhaust from, the cylinder.
In order to maximize the conversion of the energy released by combustion into motion, it is desirable to prevent heat from being conducted away from the combustion chamber through the piston. Reduction of heat lost through the piston increases the engine's operating efficiency. Typically, heat transfer through the piston is reduced or blocked by insulating the piston crown from the body of the piston. However, it is also the case that retention of the heat of combustion at the end surface of the piston can cause thermal damage to the piston crown and nearby piston elements.
Piston thermal management is a constant concern, especially given the ever-increasing loads expected from modern internal combustion engines. In a typical piston, at least four areas are of concern for thermal management: the piston crown, the ring grooves, the piston under-crown, and the piston/wristpin interface. The piston crown can be damaged by oxidation if its temperature rises above the oxidation temperature of the materials of which it is made. Mechanical failure of piston elements can result from thermally-induced material changes. The rings, ring grooves, and the lands that border the ring grooves can suffer from carbon build-up caused by oil heated above the coking temperature. As with the ring grooves, the under surface of the piston crown can also suffer from oil coking.
A recent study indicates that an opposed-piston engine two-stroke cycle engine exhibits increased thermal efficiency when compared with a conventional six-cylinder four-cycle engine. (Herold, R., Wahl, M., Regner, G., Lemke, J. et al., “Thermodynamic Benefits of Opposed-Piston Two-Stroke Engines,” SAE Technical Paper 2011-01-2216, 2011, doi:10.4271/2011-01-2216.) The opposed-piston engine achieves thermodynamic benefits by virtue of a combination of three effects: reduced heat transfer due to a more favorable combustion chamber area/volume ratio, increased ratio of specific heats from leaner operating conditions made possible by the two-stroke cycle, and decreased combustion duration achievable at the fixed maximum pressure rise rate arising from the lower energy release density of the two-stroke engine. With two pistons per cylinder, an opposed-piston engine can realize additional thermodynamic benefits with enhanced piston thermal management.